In a gallon sized zip-loc freezer bag, add olive oil, onion, garlic, Italian seasoning, oregano, dried rosemary, salt, and pepper. Seal and shake ingredients together.

Add pork to the bag and seal bag removing as much air as possible. Make sure the pork gets coated with the marinade. Place in refrigerator for at least 1 hour or longer.

Preheat oven to 425 degrees. Line a 9 x 13 inch baking dish with aluminum foil. Add the tenderloin to the lined baking dish.

Pour the marinade from the bag all over the tenderloin and bake at 425 degrees for 15 minutes.

Turn meat over and bake another 15 minutes. Turn meat over once more and bake another 15 minutes. This will cook for a total of 45 minutes.

Remove from oven and let rest for 10 minutes.

Cut into 1/4" or thicker slices and serve. Serves 4-6.